Necrotic Skin Lesions: Spider Bite—or Something Else?

Necrotic Skin Lesions: Spider Bite—or Something Else?

Patients often attribute the otherwise unexplained development of a dermonecrotic lesion to a spider bite. This self-diagnosis is rarely corroborated by evidence, however. The spider is seldom seen by the patient, let alone recovered for identification, and the bite is often not felt.

Although spider bites can produce dermonecrotic wounds, the differential diagnosis of such lesions is extensive. Most of the conditions in the differential are far more common than spider bites. Thus, we suggest that you approach the complaint of a "spider bite" with a skeptical eye.

Here we describe the clinical features of spider bites and summarize the treatment options. We also examine the reasons why the diagnosis of "spider bite" remains a popular explanation for any unexplained dermonecrotic lesion, and we review the conditions in the differential diagnosis of such lesions.

SPIDER BITES: AN OVERVIEWSpiders have complex and specialized feeding strategies; they often use webs to capture and eat insects and other small arthropods. Unlike insects that feed on human blood or serum, spiders have no reason to bite humans. Thus, a spider bite must be considered an anomaly that probably resulted from human actions which induced the arthropod to bite in self-defense.

In most cases, the medical consequences of a spider bite are minor. Typically, the bite causes limited local tissue inflammation similar to the bites and stings of other small arthropods, and patients respond well to over-the-counter analgesics and antihistamines, if any treatment is needed. Systemic effects, if they occur, are typically mild.

The bites of only a few spider species produce medically significant effects in humans. In the United States, the brown recluse (Figure 1) and the black widow (Figure 2) are the 2 best-known species that can cause serious illness.

Brown recluse spider. The brown recluse spider (Loxosceles reclusa) is endemic in the south central states, from Texas to the Carolinas, and as far north as Iowa and Illinois (Figure 3).1 Other Loxosceles species in the Southwest cause fewer reported bites, and their envenomations are typically less severe.

The brown recluse spider is tan to brown and has a darker mark on the dorsal cephalothorax that resembles a violin; hence, its other common names, violin spider and fiddleback. As its name implies, the brown recluse spider dwells in low-traffic areas, such as attics, basements, and woodpiles. In states where the spider is endemic, hundreds or even thousands of brown recluse spiders may be found in a single home, yet no one in the household has been bitten.2

Clinical features. Patients who have been bitten by a brown recluse spider may report a pinprick sensation, although the bite may be painless. Bites most commonly occur when a person disturbs a spider after he or she puts on clothes that were left on the floor or rolls over in bed onto the arthropod. Brown recluse spiders are nonaggressive toward humans; however, when they feel threatened, they may bite in self-defense.

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